1. Field of the Invention
The present invention relates to a solid state imaging device, and more particularly to a solid state imaging device capable of reducing the vertical smear and reset noise caused by an imaging element which uses a charge transfer device as horizontal scanning means.
2. Description of the Prior Art
A solid state imaging element using a two-dimensional MOS type diode array or a two-dimensional charge coupled device has been put to practical use, with the advance of the semiconductor integration circuit technology. Further, it has been proposed to use a charge injection device in a solid state imaging element. In recent years, the present inventors have proposed an imaging element which includes an MOS type diode array and a one-dimensional charge transfer device in such a manner as shown in FIG. 1, in a Japanese patent application (Application No. 194685/1983), and have pointed out that the vertical smear can be reduced by using such an imaging element.
The operation of a solid state imaging device shown in FIG. 1 will be explained below in detail. The imaging device includes a photodiode array 6, a vertical shift register 1, an interlace circuit 3, a horizontal charge transfer device 10, and circuits 14 through 17 for reducing the vertical smear.
In operation, output pulses from the vertical shift register 1 are delivered successively on output lines 2.sub.1, 2.sub.2 . . . and 2.sub.m, and are applied to the interlace circuit 3, which delivers output pulses to output lines 4.sub.1, 4.sub.3, . . . and 4.sub.2m-1 to form an odd field sequentially, and delivers output pulses to output lines 4.sub.2, 4.sub.4, . . . and 4.sub.2m to form an even field sequentially. Thus, in a horizontal scanning period for obtaining the first horizontal scanning line of the odd field, vertical switching transistors 5.sub.1-1, 5.sub.1-2, . . . and 5.sub.1-n are simultaneously turned on, and signal charges generated in photodiodes 6.sub.1-1, 6.sub.1-2, . . . and 6.sub.1-n are transferred to vertical signal lines 7.sub.1, 7.sub.2, . . . and 7.sub.n, respectively. Similarly, in a horizontal scanning period for obtaining the first horizontal scanning line of the even field, vertical switching transistors 5.sub.2-1, 5.sub.2-2, . . . and 5.sub.2-n are simultaneously turned on, and thus signal charges generated in photodiodes 6.sub.2-1, 6.sub.2-2, . . . and 6.sub.2-n are transferred to the vertical signal lines 7.sub.1, 7.sub.2, . . . and 7.sub.n, respectively. Further, horizontal switching transistors 8.sub.1, 8.sub.2, . . . and 8.sub.n are simultaneously turned on by an ON signal applied to a control terminal 9, and thus the signal charges on the vertical signal lines 7.sub.1, 7.sub.2, . . . and 7.sub.n are transferred to the charge transfer device (hereinafter referred simply to as "CTD") 10, which acts as a horizontal shift register.
In the above imaging device, it is desirable that only the photodiodes 6.sub.1-1, . . . and 6.sub.2m-n are sensitive to light. In fact, however, the surroundings of each photodiode 6, for example, the drain of each vertical switching transistor 5 may be sensitive to light. The electric charge generated in the drain of each vertical switching transistor 5 is transferred to a corresponding one of the vertical signal lines 7.sub.1, 7.sub.2, . . . and 7.sub.n, no matter whether the vertical switching transistor 5 is turned on or not. Hundreds of vertical switching transistors are arranged in a vertical direction, and the drains thereof are all connected to one vertical signal line. Accordingly, electric charges generated in these drains are accumulated on the vertical signal line. Further, the above electric charges are added to a signal charge which is generated in one of photodiodes connected to the above vertical switching transistors, at each horizontal scanning period. Thus, the so-called smear is generated. For example, when the image of an object having light portions such as shown in FIG. 2A is formed by the above-mentioned imaging device, each light portion appearing on the image is extended in a vertical direction, as shown in FIG. 2B.
The above-mentioned noise component peculiar to such an imaging device is called vertical smear. The amount of electric charge which causes the vertical smear, is proportional to a period that the electric charges generated in the drains of vertical switching transistors which are arranged in a vertical direction, are accumulated on one vertical signal line. That is, the amount of vertical smear charge which is transferred from the vertical signal line to the horizontal CTD 10 when the horizontal switching transistors 8.sub.1, 8.sub.2, . . . and 8.sub.n are turned on, is proportional to a time interval from a time these horizontal switching transistors were turned off, till a time the same transistors are turned on.
FIG. 3 shows a 3-phase electrode structure used in the horizontal CTD 10 of FIG. 1, and the internal potential corresponding to each electrode. In FIG. 3, the abscissa indicates a distance x. In parts (b) through (n) of FIG. 3, the ordinate indicates an internal potential .phi..sub.s. Further, FIG. 4 is a time chart showing pulse signals applied to 3-phase electrodes.
When the horizontal CTD 10 has a 3-phase electrode structure such as shown in part (a) of FIG. 3, the potential well corresponding to each electrode takes such a state as shown in part (b) of FIG. 3.
When pulse signals shown in parts (a), (b) and (c) of FIG. 4 and having a frequency f.sub.c are applied to three electrode groups H.sub.1, H.sub.2 and H.sub.3, respectively, the state of the potential well corresponding to each electrode is changed as shown in parts (c) through (n) of FIG. 3, in a period between t.sub.0 and t.sub.13. When those portions of the horizontal CTD 10 which correspond to the first electrode group H.sub.1, are connected to the horizontal switching transistors 8.sub.1, 8.sub.2, . . . and 8.sub.n and these horizontal switching transistors are turned on in both a period between t.sub.0 and t.sub.1 and a period between t.sub.7 and t.sub.8, two charges Q.sub.A and Q.sub.B are separately taken in potential wells which are formed beneath the electrode groups H.sub.1 and H.sub.2, as shown in part (j) of FIG. 3. Thereafter, the charges Q.sub.A and Q.sub.B are separately transferred to the output side of the horizontal CTD 10, as shown in parts (k) through (n) of FIG. 3.
FIG. 5 is a time chart for explaining the charge transfer operation of the solid state imaging device shown in FIG. 1. Further, FIG. 6 shows an output voltage signal in the imaging device of FIG. 1 and sampling pulse signals used in the above imaging device.
As shown in FIG. 5, the horizontal switching transistors 8.sub.1, 8.sub.2, . . . and 8.sub.n are turned on in a period between t.sub.a and t.sub.b, which is included in a horizontal blanking period. Thus, vertical smear charge Q.sub.v1 which has been accumulated after a period between t'.sub.e and t'.sub.f (namely, an ON-period immediately before the present horizontal scanning period) and corresponds to the charge Q.sub.A of FIG. 3, is transferred to the horizontal CTD 10. The above operation in the period between t.sub.a and t.sub.b corresponds to that operation in the period between t.sub.1 and t.sub.2 which is shown in part (c) of FIG. 3. Thereafter, the vertical switching transistors 5.sub.i-1, 5.sub.i-2, . . . and 5.sub.i-n (where i=1, 2, . . ., or 2m) are turned on in a period between t.sub.c and t.sub.d, to transfer signal charges Q.sub.S generated in the photodiodes 6.sub.i-1, 6.sub.i-2, . . . and 6.sub.i-n to the vertical signal lines 7.sub.1, 7.sub.2, . . . and 7.sub.n. Then, the horizontal switching transistors 8.sub.1, 8.sub.2, . . . and 8.sub.n are again turned on in a period between t.sub.e and t.sub.f to transfer each signal charge Q.sub.S (corresponding to the charge Q.sub.B of FIG. 3) to the horizontal CTD 10. An operation in a period between t.sub.b and t.sub.e corresponds to those operations in a period between t.sub.2 and t.sub.7 which are shown in parts (d) to (h) of FIG. 3, and the operation in the period between t.sub.e and t.sub.f corresponds to that operation is a period between t.sub.7 and t.sub.8 which is shown in part (i) of FIG. 3. Further, when those operations in a period between t.sub.8 and t.sub.13 which are shown in parts (j) to (n) of FIG. 3, are repeated without turning on the horizontal switching transistors 8.sub.1, 8.sub.2, . . . and 8.sub.n, the vertical smear charge Q.sub.v1 and signal charge Q.sub.S can be separately transferred through the horizontal CTD 10.
But according to these operation the signal charge Q.sub.S is combined with vertical smear charge Q.sub.v2 which is accumulated in a period between t.sub.b and t.sub.f. Now, let us express a period between t'.sub.f and t.sub.b and the period between t.sub.b and t.sub.f by T.sub.1 and T.sub.2, respectively. Further, one horizontal scanning period is about 64 .mu.s, and the horizontal blanking period is about 11 .mu.s. Accordingly, the relation between the charge Q.sub.v1 and the charge Q.sub.v2 is given as follows: ##EQU1##
Owing to the above-mentioned operation, the vertical smear charge Q.sub.V1 and the signal charge Q.sub.S combined with the vertical smear charge Q.sub.v2 are alternately delivered from the output end of the horizontal CTD 10. The smear charge Q.sub.V1 and the combined charge Q.sub.S +Q.sub.V2 are converted by the gate capacitance C.sub.G of a source followered transistor 11 into voltage signals V.sub.1 and V.sub.2, which appear on an output terminal 12. The voltage signals V.sub.1 and V.sub.2 are given by the following equations: ##EQU2##
A reset transistor 13 which is turned on by a reset pulse signal having the same frequency as a clock frequency f.sub.c, is connected to the gate of the transistor 11, to sweep out the charge at the gate of the transistor 11 before the next charge is transferred from the horizontal CTD 10 to the above gate. A noise charge Q.sub.n is generated by the ON-OFF action of the reset transistor 13, and is held on the gate of the transistor 11 till the next ON-OFF action of the reset transistor 13. The noise charge is converted into a voltage, and appears on the output terminal 12 in the form of reset noise. As a result, final output voltages V.sub.1 ' and V.sub.2 ' appear alternately on the output terminal 12, are given by the following equations: ##EQU3##
Examples of the output voltages V.sub.1 ' and V.sub.2 ' are shown in FIG. 6. These output voltage V.sub.1 ' and V.sub.2 ' are alternately obtained at the output terminal 12 by applying the third electrode group H.sub.3 with clock pulses, which have the repetition frequency f.sub.c. The output voltages V.sub.1 ' and V.sub.2 ' thus obtained are separated from each other, by sampling circuits 14.sub.1 and 14.sub.2 which are shown in FIG. 1. Sampling pulse signals SP1 and SP2 which are shown in FIG. 6, are applied to the sampling circuits 14.sub.1 and 14.sub.2, respectively, and thus the circuits 14.sub.1 and 14.sub.2 take in the output voltage V.sub.1 ' and V.sub.2 ', respectively. The output voltage V.sub.1 ' extracted by the sampling circuit 14.sub.1 is attenuated by a gain control circuit 15 to a value equal to the product of a factor T.sub.2 /T.sub.1 and the voltage V.sub.1 ', and then applied to a subtractor 16 together with the output voltage V.sub.2 ' which is extracted by the sampling circuit 14.sub.2. Accordingly, an output signal V.sub.out from the subtracter 16 is given by the following equation: ##EQU4##
In the above equation (6), Q.sub.V2 is equal to (T.sub.2 /T.sub.1)Q.sub.V1 according to equation (1). Therefore, the equation (6) is rewritten as follows: ##EQU5##
The output signal of the subtracter 16 is further applied to a low pass filter 17. As is evident from the equation (7), in the output of the filter 17, the vertical smear is extinguished, and the reset noise due to noise charge Q.sub.n1 combined with the smear charge Q.sub.V1 is reduced by a factor T.sub.2 /T.sub.1. However, the reset noise due to noise charge Q.sub.n2 combined with the signal charge Q.sub.S is left as it is. Since any correlation does not exist between the noise charges Q.sub.n1 and Q.sub.n2, the equation (7) can be rewritten as follows: ##EQU6##